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Author Topic: proper size transformer to power a 28v dynamotor in a BC-348  (Read 2501 times)
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KG4KGL
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« on: December 03, 2023, 01:38:24 PM »

Hello all,

I was recently given a two BC348 receivers. This had moreto do with a timely response, and most importantly, I was local as the owner did not want to ship them.

 One is an "O" and the other is an "R". One has a homemade DC supply with a relay. I assume that was to break the B+, when the transmitter was being keyed.

The other has the dynamotor. I want to power the dynamotor with a 28Vdc supply. My question is, what is the recommended current capacity for a 120-28vac transformer?

Thank you,

Robert
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K8DI
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« Reply #1 on: December 03, 2023, 03:14:20 PM »

So a quick search says you need a little under 2 amps run, but 8-10 amps to start the motor, plus whatever the filaments, etc., need.   If you get a dual secondary 10v filament transformer, wire the two secondaries in series, you'll have 20v, which after bridge rectification, will be 28 volts.  You could also buy a new 20v transformer. Antek makes one.  Take the 2 amps, plus the filaments, double it, for a decent margin. Use big diodes for the bridge, 20 amp or so, so the starting surge is well under their limit. Use an oversized filter cap, which will help provide that surge to start the motor. Switch the power supply on then turn on the radio/dynamotor so the cap is charged.  No worries about the transformer not being surge rated -- cold filaments have large inrush currents, too...

Ed
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KA3EKH
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« Reply #2 on: December 03, 2023, 04:05:09 PM »

Check eBay; look for a used 24 volt 10 or 13 amp power supply. It may cost less then the transformer. Not a big fan of using the dynamotor, lot of people are into this whole thing about keeping it one hundred percent original but at the end of the day its noisy and tends to require maintenance and adds nothing to the experience of using the radio, but thatís me so as many will tell you I am the last one to tell anyone what to do.
There is another thread over in the military surplus section thatís got the schematic and information on the filament string if you wanted to change it to AC power.
I kind of like having the radio configured like it would have been use by those Old Timer AM operators  in the late forties and fifties being a lot more of those receivers served in the Ham and SWL community then the short period of time they were in military service.

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KG4KGL
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« Reply #3 on: December 03, 2023, 05:37:31 PM »

Check eBay; look for a used 24 volt 10 or 13 amp power supply. It may cost less then the transformer. Not a big fan of using the dynamotor, lot of people are into this whole thing about keeping it one hundred percent original but at the end of the day its noisy and tends to require maintenance and adds nothing to the experience of using the radio, but thatís me so as many will tell you I am the last one to tell anyone what to do.
There is another thread over in the military surplus section thatís got the schematic and information on the filament string if you wanted to change it to AC power.
I kind of like having the radio configured like it would have been use by those Old Timer AM operators  in the late forties and fifties being a lot more of those receivers served in the Ham and SWL community then the short period of time they were in military service.



I am not concerned with keeping it 100% period correct. I just want to try the Dynamotor for curiosity's sake. I do not plan on making any changes that can not be undone. There will not be any drilling on the case.

I am planning on using this for CW only at this time, with a DX-20 transmitter. This would be some what period correct for a novice setup. I also have a BC-453 that could be paired with the 348 as a Q multiplier. That looks like fun as well.

Robert
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KA3EKH
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« Reply #4 on: December 03, 2023, 08:33:59 PM »

Dynamotors being mechanical are fun to play with but when you start to have to deal with inrush current to get them going and bearings, vibrations and brushes get old fast! Back when I took the test for the second radiotelephone they still had a bunch of questions regarding the care and feeding of dynamotors that was in 78 and by then no one was using them in two way but still they had at least five questions. They also had questions regarding wet cell batteries!
I think that in the last ten or so yearís people have all gone Gaga over wanting and using them. You can get more for the BC-348 dynamotor on eBay then the radio itself because all the collectors feel their life wonít be complete without one. All the Ham removed them and built AC power supplies so not many around today. Year ago I did a video on YouTube about converting a BC-348 with the common Ham modifications and got hate mail from people about ďhow dare can you do thatĒ the weird thing is that receiver in the video I recently went back thru and did install a all original face plate but still have the AC supply and a audio output transformer thatís low impedance.
My first SW receiver back when I was thirteen years old was a BC-348 although I always preferred the Q model with newer tubes. I currently have that receiver paired with an ARC-5 80 meter transmitter for my CW station, biggest problem is my CW skills suck.

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w7fox
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« Reply #5 on: December 03, 2023, 09:05:44 PM »

Robert,
I have used a 453 as a Q-5er, but the 453 doesn't tune to the 348's IF without modifying the coils in the 453.  It can be done, but requires some work.  I probably saw the modification in one of the hints and kinks books.

I bought a BC348 in 1976 at a local aircraft recycler.  It was fun to run on the dynamotor, and as each capacitor in the receiver failed, I could hear the dynamotor groan and slow down.  Luckily, I was quick to turn off the power supply before any damage occurred.  After this happened twice, I replaced all the caps!  No trouble after that.  Good luck, and enjoy the BC348.
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KG4KGL
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« Reply #6 on: December 05, 2023, 11:18:58 AM »

Robert,
I have used a 453 as a Q-5er, but the 453 doesn't tune to the 348's IF without modifying the coils in the 453.  It can be done, but requires some work.  I probably saw the modification in one of the hints and kinks books.


SA2CLC has a YT vid of a BC-348 and BC-453, that he says was not modified. He takes the signal from the 3rd IF of the 348 and feeds the mixer of the 453.

His math is as follows:

"The heptode mixer tube mixes on overtones as well as fundamental freqs.

f=(915+/-85)/2
f=500 or 375
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SM6OID
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« Reply #7 on: December 06, 2023, 01:48:38 AM »

Hi!

The BC-348-Q with dynamotor that I have pulls slightly above 2 A @ 26 V with dial lights at full brightness.
Starting current as mentioned, is another thing.
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n8fvj
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« Reply #8 on: December 06, 2023, 09:57:10 AM »

A motor does not need filtered DC, just DC. This transformer into a 25 amp bridge rectifier will start and run the dynamotor for $40. AS-2228 - 200VA 28V TRANSFORMER The transformer is rated at 50Hz, has 20% more ampacity at 60Hz and would surge more than the 20% more amps or up to 10 amps for over 30 seconds.
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Patrick J. / KD5OEI
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« Reply #9 on: December 06, 2023, 03:42:04 PM »

The amount of ripple from a power supply being used to run a dynamotor will modulate the dynamotor's output and cause ripple to appear there.

This effect varies among dynamotors and % load, etc. The ripple coming from a dynamotor may be a little less than the ripple going in, but not by much. There is inductance of the field and rotor windings but it's not enough to act as a filter in itself. Raw rectifier output from a sine wave (or any non-square-wave) AC power source is unsuitable as a power supply for a dynamotor from which smooth DC for radios is expected.
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n8fvj
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« Reply #10 on: December 07, 2023, 09:30:43 AM »

The amount of ripple from a power supply being used to run a dynamotor will modulate the dynamotor's output and cause ripple to appear there.

This effect varies among dynamotors and % load, etc. The ripple coming from a dynamotor may be a little less than the ripple going in, but not by much. There is inductance of the field and rotor windings but it's not enough to act as a filter in itself. Raw rectifier output from a sine wave (or any non-square-wave) AC power source is unsuitable as a power supply for a dynamotor from which smooth DC for radios is expected.

A dynamotor is a motor and generator. The motor spins at a constant speed if filtered AC or not filtered AC and the generator makes DC output. How can the AC ripple appear on the generator output? It cannot.
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K8DI
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« Reply #11 on: December 07, 2023, 09:59:08 AM »

The amount of ripple from a power supply being used to run a dynamotor will modulate the dynamotor's output and cause ripple to appear there.

This effect varies among dynamotors and % load, etc. The ripple coming from a dynamotor may be a little less than the ripple going in, but not by much. There is inductance of the field and rotor windings but it's not enough to act as a filter in itself. Raw rectifier output from a sine wave (or any non-square-wave) AC power source is unsuitable as a power supply for a dynamotor from which smooth DC for radios is expected.

A dynamotor is a motor and generator. The motor spins at a constant speed if filtered AC or not filtered AC and the generator makes DC output. How can the AC ripple appear on the generator output? It cannot.

Ahh, but, you assume that a motor is a continuous constant speed device. It isn't.  Simple case, a motor with a pair of brushes and a commutator.  As it spins, different coils are connected via the brushes to electricity.  So they are going on and off -- and, being an inductor, there is a rise time and a fall time for the current and magnetic field. It pulses, ebbs and flows...with a an unfiltered supply, it also does the same thing DURING the time it is connected to each coil.  The vibrations or modulation of the shaft speed is very real, but quite small.  A flywheel lessens it. But, remember, the dynamotor is small and light. Now about ripple...for a quiet source of power, we shoot for 1% or less.  1% is a pretty small variation...easy to see how this could happen in a dynamotor just from the commutation, as well as the varying power.  Add to that the mixing/beating of those two creating products that add to the total ripple/variation as well.

Ed
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n8fvj
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« Reply #12 on: December 07, 2023, 10:35:12 AM »

The dynamotor does have a constant speed with unfiltered DC. No way can the motor respond to 120hz ripple. However, if the owner wants filtered DC this transformer will work. Antek Transformer AS-2222 - 200VA 22 Volt.
Use a 25 amp bridge rectifier into a 22000uF 35 volt capacitor for about 27 volts output.
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Patrick J. / KD5OEI
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« Reply #13 on: December 07, 2023, 11:37:55 PM »

It does not respond at the ripple frequency in terms of RPM. That was not the point.
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n8fvj
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« Reply #14 on: December 08, 2023, 01:13:44 AM »

It does not respond at the ripple frequency in terms of RPM. That was not the point.
[Steady RPM, steady ripple free DC output./quote]
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KA3EKH
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« Reply #15 on: December 08, 2023, 08:52:49 AM »

Dynamotors are a dead technology, and not without good reason. But looks like you want to get it running and experience it for yourself. I was looking around on the internet and saw this web page:

https://www.instructables.com/reviving-a-WW2-dynamotor-how-tube-radios-went-mob/

information about how to clean and lubricate a BC-348 dynamotor and donít know if you saw this or not but thought it may be useful. Attached a picture of my 348Q, whats yours look like?



* IMG_1630.JPG (1387.67 KB, 2420x1815 - viewed 70 times.)
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KA2DZT
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« Reply #16 on: January 06, 2024, 10:15:51 PM »

I have a BC-348J  I've had the receiver for about 58 years  I build a power supply for it back then and it still works.  I removed the dynamotor and mounted the supply where the dynamotor was.  I also had to rewire the filaments.

Fred
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MikeKE0ZUinkcmo
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« Reply #17 on: January 13, 2024, 12:58:22 AM »

I bought mine about 50 years ago and after maybe 30 or 40 hours, replaced the dynamotor with an AC supply.    Though initially kinda Cool, after not too many hours whine gets real annoying.



I used two transformers, one for the B+, and the other for the 28V filaments, lots easier than rewiring everything.
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Mike KE0ZU

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